Pacific gaint salamenders in Mack Creek: Size and weight vs. channel location in old growth and clear cut forest section
Shuhan Song
November 23, 2019
Introduction
Data and Methods
Results
A. Visually compare annual salamander counts in old growth (OG) and clear cut (CC) sections of Mack Creek.
Figure 1: Annual count of Pacific gaint salamanders in CC or OG sections of Mack Creek (1993-2017). In both sections, the count of Pacific gaint salamanders generally increased from 1993 to 2011, dropped from 2011 to 2014, then increased again until 2017. In most years, salamanders in old growth sections have greater counts than those in clear cut sections. Data source: Andrews Forest LTER Site
B. Salamander counts by channel classification and sections of Mack Creek in 2017.
Table 1: Numbers of salamanders within Mack Creek in 2017. Values indicated are counts and proportions (by sections) in channel units (cascades, pool, and side-channel). In CC section, 67.1% observations are from cascades (n = 247), 8.4% from pools (n = 31), and 24.5% from side channels (n = 90). In OG section, 62.8% observations are from cascades (n = 201), 14.1% from pools (n = 45), and 23.1% from side channels (n = 74). Data source: Andrews Forest LTER Site
| Sections | Cascades | Pool | Side-channel |
|---|---|---|---|
| CC | 247 (67.1%) | 31 (8.4%) | 90 (24.5%) |
| OG | 201 (62.8%) | 45 (14.1%) | 74 (23.1%) |
C. Is there a significant difference in where in the channel Pacific giant salamanders are located between the two forest condition (OG and CC)?
There is no significant effect of forest conditions (OG/CC) on where in the channel salamanders are located (cascades/pool/side-channel) (\(\chi\)2(2) = 5.54, p = 0.06). Salamanders from old growth and clear cut sections in the forest have no different chances to be spotted in cascades (OG = 201 (62.8%), CC = 247 (67.1%)), pool (OG = 45 (14.1%), CC = 247 (67.1%)), and side-chanel (OG = 74 (23.1%), CC = 90 (24.5%)).
D. Compare weights of Pacific giant salamanders in OG and CC forest sections of the creek in 2017.
Although weight in both forest sections (OG and CC) are positively skewed, the large number of data (OG = 320, CC = 368) allow us to use t-test to compare mean (Figure 2).
Figure 2: The weight of Pacific gaint salamanders in forest sections (CC/OG) in 2017. The distributions of weight in both forest sections are positively skewed. Salamanders in CC forest section have weight 7.78\(\pm\) 9.9 gram (n = 368) while those in OG sections have weight 6.7\(\pm\) 9.04 gram (n = 320). Data source: Andrews Forest LTER Site
For 2017 observations, weight (gram) of Pacific gaint salamanders measured in samples collected from OG sections (6.7 \(\pm\) 9.04, n = 320) differed significantly from salamander weight in CC sections (7.78 \(\pm\) 9.9, n = 368) by a two-sample t-test (t(683) = 1.49, p = 0.138, 95% CI = (-0.35, 2.49)).
Based on p-value, there was a 13.8% chance that we could have found two samples at least as different as ours if they were drawn from two populations with the same means, which is not signifcantly low.
In addition, if we took many samples and found the confidence intervals, we’d expect 95% of those confidence intervals to included the population mean differece. Therefore, it’s highly likely that a calculated confidence interval contains the population mean difference, where 0 was included. The difference in salamaders weight between OG and CC forest section was 1.08mm, with a negligible effect size (Cohen’s d = -0.11).
Therefore, we failed to reject the null hypothesis that the weight of Pacific gaint salamanders measured in OG and CC forest sections were the same. The difference in forest conditions did not impact salamanders’ weight in Mack Creak samples.